1. Field of the Invention
The present invention relates to a current-limited protection circuit of a switching power converter, and more particularly to an amplifier for current-limited protection circuit of a switching power converter.
2. Description of the Related Art
FIG. 1 is a schematic circuit of a prior art switching power converter. Referring to FIG. 1, the first source/drain terminal of the switch transistor 101 is coupled to the input terminal of the switching power converter for receiving the input voltage Vin. The second source/drain terminal is coupled to the output terminal of the switching power converter via the series inductor 103a and resistor 103b. A terminal of the diode 107 is coupled to the second source/drain of the switch transistor 101, and another terminal of the diode 107 is grounded via the resistor 109. The output terminal of the switching power converter is grounded via the capacitor 111. The output voltage Vo drops on the two terminals of the capacitor 111.
Referring to FIG. 1, the pulse-width modulator 130 comprises a current-limited comparator 131 having the positive-voltage and the negative-voltage terminals coupled to the two terminals of the resistor 109. The positive-voltage input terminal of the current-limited comparator 131 is coupled to a terminal of the resistor 109 via the reference voltage Vf. The current-limited comparator 131 is coupled to the direct current (DC) voltage Vc so that the pulse-width modulator 130 operates normally. The output terminal of the current-limited comparator 131 is coupled to the gate terminal of the switch transistor 101 via the output terminal of the current-limited comparator 131.
In the switching power converter of FIG. 1, the voltage Vr dropped on the resistor 109 depends on the current flowing therethrough. When the working current I flowing through the switch transistor 101 is high, the current flowing through the resistor 109 also increases. When the voltage Vr dropped on the resistor 109 is higher than a preset voltage, the pulse-width modulator 130 turns off the switch transistor 101. The current I flowing through the switching power converter declines so that circuit protection can be achieved.
However, the prior art switching power converter has disadvantages. In order to resolve the heat dissipation problem, the size of the resistor 109 should be increased. Due to the increase of the size of the resistor 109, the size of the integrated circuit also increases. In order to prevent reducing the resistance of the resistor 109 resulting from the power dissipation, the resistance of the resistor 109 should be smaller. For fabricating a resistor with a precise and small resistance, the manufacturing costs increase.
U.S. Pat. No. 5,982,160 (Walters et. al.) discloses a DC-to-DC converter with inductor current sensing and related methods. The method comprises: sensing current passing through the inductor using a current sensor connected in parallel with the inductor, the current sensor comprising a resistor and a capacitor connected together in series; and operating the peak current control loop circuit to control the at least one power switch responsive to the current sensor. The method applies a DC voltage and AC voltage for the induced voltage, wherein the DC voltage is higher than the AC voltage. While the induced voltage is amplified, the DC voltage should be filter out and the AC voltage should be amplified due to its low value. The method is complicated.